[llvm][IfConversion] update successor list when merging INLINEASM_BR

If this successor list is not correct, then branch-folding may
incorrectly think that the indirect target is dead and remove it. This
results in a dangling reference to the removed block as an operand to
the INLINEASM_BR, which later will get AsmPrinted into code that doesn't
assemble.

This was made more obvious by, but is not a regression of
https://reviews.llvm.org/D130316.

Fixes: https://github.com/llvm/llvm-project/issues/60346

Reviewed By: efriedma, void

Differential Revision: https://reviews.llvm.org/D142924

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index 105ab90..936de31 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -2244,6 +2244,15 @@ void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
   assert(!FromMBB.hasAddressTaken() &&
          "Removing a BB whose address is taken!");
 
+  // If we're about to splice an INLINEASM_BR from FromBBI, we need to update
+  // ToBBI's successor list accordingly.
+  if (FromMBB.mayHaveInlineAsmBr())
+    for (MachineInstr &MI : FromMBB)
+      if (MI.getOpcode() == TargetOpcode::INLINEASM_BR)
+        for (MachineOperand &MO : MI.operands())
+          if (MO.isMBB() && !ToBBI.BB->isSuccessor(MO.getMBB()))
+            ToBBI.BB->addSuccessor(MO.getMBB(), BranchProbability::getZero());
+
   // In case FromMBB contains terminators (e.g. return instruction),
   // first move the non-terminator instructions, then the terminators.
   MachineBasicBlock::iterator FromTI = FromMBB.getFirstTerminator();